Open Mobile Alliance Broadcast (also known as OMA BCAST) is an international standard wherein, amongst others, conditional access (CA) systems are specified for use with mobile devices that are capable of receiving digital broadcast streams, such as digital TV streams. The OMA BCAST specification describes a CA system that enables access to a broadcast transmission for a period between two defined times, i.e. between a start time with timestamp Ts and an end time with timestamp Te. The part of the broadcast transmission between the start and end time is called an event. The current time is transmitted in cryptograms to mobile devices as timestamp Tc. Typically this involves sending short term key messages (STKM) containing the cryptograms. These cryptograms also contain a descrambling key (also known as a control word or CW) for descrambling the broadcast stream. Successive timestamps Tc are transmitted in successive cryptograms to update the current time in the mobile device. If the mobile device is entitled to watch an event, e.g. in case of a subscription to the event, the timestamps Ts and Te identifying the start en and time of the event in the broadcast stream are transmitted to the mobile device. Typically this involves sending a long term key message (LTKM) containing the Ts and Te values. The mobile terminal is entitled to descramble the broadcast stream if Tc falls within the range Ts-Te.
In the OMA BCAST specification, events are predefined by defining timestamps Ts and Te for the events in a broadcast stream. In practice an event can have a variable start and end time. An example of this is a sport event that is undecided at the scheduled end time of the regular game play and an extension of the game is needed to determine the winner. A fixed time period for events as implemented by OMA BCAST does not allow entitlements to be associated with events that have a flexible schedule or a variable duration. In the OMA BCAST system, simply extending the allowed period of access to the broadcast stream to accommodate any variations in start and/or end time of an event is undesirable, as illustrated in the following example.
Assume that the broadcast stream contains a first event being a sports event that is scheduled from Ts1=0 to Te1=1500 and a second event being a movie that is scheduled from Ts2=1501 to Te2=3000. A first user is entitled to watch the sports event, but not the movie. Hereto, the mobile device of the first user receives a LTKM containing Ts1 and Te1 defining the start time and end time of the sports event in the broadcast stream and allowing decryption of the broadcast stream during this time period. A second user is entitled to watch the movie, but not the sports event. Hereto, the mobile device of the second user receives a LTKM containing Ts2 and Te2 defining the start time and end time of the movie in the broadcast stream and allowing decryption of the movie during this time period. A timestamp Tc indicating the current time and a CW for decrypting the broadcast stream are transmitted in a STKM to the mobile devices at a predefined time interval, e.g. 10 times per second. Each mobile device determines if the current time Tc is within an entitled time frame Ts-Te and if this is the case the CW is used to descramble the broadcast stream. In case of extending the sports event, updating Te1 to a later time—e.g. to Te1=1800—and broadcasting the sports event accordingly, enables the second user to watch the sports event starting from Tc=1501 until Tc=1800 as this time period falls within Ts2 and Te2. The problem is cascaded to further events as the second event needs to be extended to Te2=3300 to allow the second user to watch the whole movie.
The OMA BCAST specification allows updating all Te and Ts values for all events and all users in the broadcast system. This is undesirable though, because the required LTKM messages for updating these timestamps result in processing overhead and increased network traffic.